1. Field of the Invention
The present invention relates generally to boiling water reactors and, more particularly, to a method and apparatus for replacing cracked core spray supply piping in a boiling water reactor.
2. Discussion of the Related Art
A typical boiling water nuclear reactor 10, as illustrated in FIG. 1, includes a reactor vessel 12, a core 14 made up of a plurality of fuel assemblies 16, and a core shroud 18 disposed concentrically within the reactor vessel around the core. Core shroud 18 includes upper and lower cylindrical sections 20 and 22 connected by a horizontal plate 24 extending radially inward from a bottom edge of the upper cylindrical section to a top edge of the lower cylindrical section. A shroud head flange is welded to the top edge of the upper cylindrical shroud section and extends radially inward to support a shroud head or lid 26 of generally hemispherical configuration, the lid being secured to the top of the shroud with bolts threadedly or otherwise engaged by lugs mounted in angularly spaced relation about the shroud periphery adjacent the top edge of the shroud.
Fuel assemblies 16 are supported at the bottom by a core plate 28 mounted on a core plate support ring extending radially inward from the bottom edge of the lower cylindrical shroud section and at the top by a top guide 30 mounted on horizontal plate 24. Control rod guide tubes 32 are provided within vessel 12 at locations above a control rod driving mechanism extending through nozzles located at the bottom of the reactor vessel beneath the shroud. Lower ends of corresponding control rods are detachably connected to the driving mechanism and are arranged to move up and down within the guide tubes.
Feedwater enters the reactor vessel via a feedwater inlet 34 and is distributed circumferentially within the reactor vessel by a ring-shaped pipe 36 disposed above the shroud and known as a feedwater sparger. The feedwater mixes with other water coming from the steam separators and flows downwardly from feedwater sparger 36 through the downcomer annulus 38, that is, the annular region between the reactor vessel and the core shroud, and ultimately enters the core lower plenum 40. A portion of the other downcomer water and feedwater is withdrawn from the reactor vessel via a recirculation water outlet 42 and is fed under pressure into a plurality of jet pump assemblies 44 distributed circumferentially about the core shroud to produce a forced convection flow through the core. Boiling is produced in the core creating a mixture of water and steam which enters the core upper plenum, that is, the space under the shroud sealing lid, and is directed into steam plenum heads or stand pipes 46 mounted vertically on the shroud sealing lid in fluid communication with the core upper plenum. The mixture of water and steam flows through stand pipes 46 and enters a respective plurality of steam separators 48, which are shown as being of the axial-flow centrifugal type. The separated liquid water then mixes with incoming feedwater and flows downwardly to the core via the downcomer annulus. The steam, on the other hand, passes through a steam drying assembly or dryer 50 disposed above the steam separators and is withdrawn from the reactor vessel via a steam outlet 52.
In a loss-of-coolant accident, or LOCA, rupturing of the recirculation duct system or the steam duct system during operation can cause coolant water to flow out of the reactor vessel thereby lowering the water level in the reactor vessel and exposing the core such that the fuel assemblies may become overheated and damaged. In order to prevent overheating of the reactor core during a LOCA, tubular core spray spargers 54 of semi-circular configuration are oriented horizontally within the upper cylindrical section of shroud 18 above top guide 30 and are apertured at multiple locations to supply water to the core. These semi-circular core spray spargers are arranged in opposed pairs to form circular rings at two elevations, with core spray inlet or supply piping 56 connecting upper and lower pairs of core spray spargers with nozzles formed in the reactor vessel above shroud 18 at respective azimuthal locations. The connection at the core spray nozzle is made with a safe end assembly having a hollow, cylindrical safe end welded to the nozzle externally of the reactor vessel and a thermal sleeve which extends from the safe end in an inboard direction, i.e., toward the interior of the reactor vessel, to a flow divider or T-box 58 disposed in the reactor vessel above the shroud. As best seen in FIG. 2, core spay supply piping 56 includes a pair of horizontal sections or arms 60 which extend circumferentially, in opposite directions, from T-box 58 to a pair of upper elbows 62 where the piping turns downwardly to connect with a pair of vertical sections 64. Each vertical section 64 of the piping extends downwardly from one of the upper elbows to a lower elbow 66 where the piping turns inwardly, or in the inboard direction, to penetrate through the shroud and connect with respective core spray spargers 54 disposed therein.
After periods of use, intergranular stress corrosion cracking of the core spray spargers and other sections of the core spray supply piping tends to occur as a result of corrosion, radiation and stress. The cracks usually occur in the heat-affected zones of the welds that join the typically austenitic stainless steel piping and associated components of the core spray supply system and are predominantly circumferential, with axial cracks occurring less frequently. Such cracking can lead to crack opening widths which permit significant leakage from the core spray spargers and the core spray supply piping. Leakage from the core spray spargers inside the shroud is typically not considered to be a major problem; however, when significant leakage from the core spray supply piping occurs outside the shroud, the piping must either be replaced or repaired.
A first type of repair involves welding the cracked piping to close the crack; however, weld repairs below the field welds connecting the vertical piping sections with the lower elbows are typically not practical due to inaccessibility, radiation exposure, and helium cracking due to radiation damage. U.S. patent application Ser. No. 08/758,056, the disclosure of which is incorporated herein by reference, discloses another method of repairing cracked core spray supply piping in a boiling water reactor wherein clamps are secured to lengths of core spray supply piping on opposites sides of a crack and are urged together to apply an axially compressive force or preload on the piping to close the crack. While this second type of repair is satisfactory for most locations, it is generally more complicated and expensive for pipe cracking which occurs below the field weld due to the complex geometry and relatively inaccessible nature of this joint and the possibility of pipe leakage due to large seismic displacements of the shroud.